2003 sprinter 313.cdi viscous fan continually runs when up to temp?

the viscous fan does not come on till the engine temp reaches 80 deg. once it comes on it runs continually,the temp remains constant at 80 deg and the engine doesnt overheat. I have checked that the rad is clear but with the fan continually running it raises the noise level in the cab Ihave had this problem for the last six months